In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), perovskite-based solar cell (PSC) devices, organic photodetectors (OPDs), organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example of between 50 and 300 nm thickness.
One particular area of importance is organic photovoltaics (OPV). Conjugated polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies above 10%.
Another particular area of importance is OFETs. The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is stable to oxidation i.e. it has a high ionization potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are good processibility, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
Organic photodetectors (OPDs) are a further particular area of importance, for which conjugated light-absorbing polymers offer the hope of allowing efficient devices to be produced by solution-processing technologies, such as spin casting, dip coating or ink jet printing, to name a few only.
The photosensitive layer in an OPV or OPD device is usually composed of at least two materials, a p-type semiconductor, which is typically a conjugated polymer, an oligomer or a defined molecular unit, and an n-type semiconductor, which is typically a fullerene or substituted fullerene, graphene, a metal oxide, or quantum dots.
However, the OSC materials disclosed in prior art for use in OE devices have several drawbacks. They are often difficult to synthesis or purify (fullerenes), and/or do not absorb light strongly in the near IR (infrared) spectrum>700 nm. In addition, other OSC materials do not often form a favorable morphology and/or donor phase miscibility for use in organic photovoltaics or organic photodetectors.
Therefore there is still a need for OSC materials for use in OE devices like OPVs, OPDs and OFETs, which have advantageous properties, in particular good processibility, high solubility in organic solvents, good structural organization and film-forming properties. In addition, the OSC materials should be easy to synthesize, especially by methods suitable for mass production. For use in OPV cells, the OSC materials should especially have a low bandgap, which enables improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, high stability and long lifetime. For use in OFETs the OSC materials should especially have high charge-carrier mobility, high on/off ratio in transistor devices, high oxidative stability and long lifetime.
It was an aim of the present invention to provide new OSC compounds, especially n-type OSCs, which can overcome the drawbacks of the OSCs from prior art, and which provide one or more of the above-mentioned advantageous properties, especially easy synthesis by methods suitable for mass production, good processability, high stability, long lifetime in OE devices, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials and n-type OSCs available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing compounds comprising a central polycyclic unit as shown in formula I discussed above.
It has been found that compounds comprising such a central polycyclic unit, and further comprising two terminal electron withdrawing groups, can be used as n-type OSCs which show advantageous properties as described above.
Conjugated polymers based on linearly fused polycyclic aromatic units have been disclosed in prior art for use as p-type OSCs, such as indacenodithiophene (IDT) as disclosed for example in WO 2010/020329 A1 and EP 2075274 A1, or indacenodithienothiophene (IDTT) as disclosed for example in WO 2015/154845 A1.
OSC small molecules with an IDT core have been proposed for use as chromophores in OLEDs by K-T. Wong, T-C. Chao, L-C. Chi, Y-Y. Chu, A. Balaiah, S-F. Chiu, Y-H. Liu, and Y. Wang, Org. Lett., 2006, 8, 5033.
More recently, OSC small molecules comprising an IDT or IDTT core that is end capped with 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile have been reported for use as non-fullerene n-type OSCs in OPV devices, for example by Y. Lin, J. Wang, Z.-G. Zhang, H. Bai, Y. Li, D. Zhu and X. Zhan, Adv. Mater., 2015, 27, 1170, and by H. Lin, S. Chen, Z. Li, J. Y. L. Lai, G. Yang, T. McAfee, K. Jiang, Y. Li, Y. Liu, H. Hu, J. Zhao, W. Ma, H. Ade and H. Yan, Zhan, Adv. Mater., 2015, 27, 7299, in CN104557968A and CN105315298 A.
However, the compounds as disclosed and claimed hereinafter have hitherto not been disclosed in prior art.